The present invention relates generally to the field of molding elastomeric articles with apparatus incorporating, for example, an injection unit and a mold, and more particularly to a venting system for evacuating fluid such as air or the like entrapped in the mold cavities preparatory to filling the latter with elastomeric stock from the injection unit.
Generally speaking, apparatus for molding elastomeric articles constituted of, for example, elastomers in the form or rubber mixtures that can be vulcanized when subjected to heat, or synthetic plastics that can be cross-linked and hardened, may be comprised of transfer molding equipment including an injection unit and a mold movable relative to one another. The mold may be comprised of a large number of cavities, whereas, the injection unit may be comprised of a cylinder having an open-ended portion from which elastomeric stock can be expelled into the various mold cavities when the mold and the injection cylinder are engaged with one another.
The injection cylinder may be provided with a multi-apertured end plate that is either fixedly secured to the open-ended portion thereof, or is frictionally retained or otherwise constrained in the open-ended portion thereof for movement axially relative thereto at least partially into the cylinder chamber. Such apparatus is generally disclosed in U.S. Pat. No. 3,859,024, issued on Jan. 7, 1975, the subject matter of which is fully incorporated by reference herein.
Briefly, in operation of a multi-cavity mold and an injection cylinder to which is fixedly secured a multi-apertured end plate, the mold and injection cylinder are moved relative to one another to effect engagement of the mold with the end plate such that the apertures of the end plate are coaxially aligned with corresponding ones of the mold cavities. Thereafter, a piston movably confined in the injection cylinder is operated to compact or compress the elastomeric stock in the injection cylinder and cause the expulsion of such stock through the apertures in the end plate and into the mold cavities.
Such operation of an injection unit to which it is fixedly secured a multi-apertured end plate is to be distinguished from the operation of an injection cylinder to which is movably secured a multi-apertured end plate. In the latter regard, the piston in the injection cylinder can be protracted slightly to force a portion of elastomeric stock axially outwardly of the injection chamber and, thereby, force at least partially outwardly therefrom the multi-apertured end plate. With the apertured end plate so outwardly disposed, and with the piston held stationary, the mold and injection unit can be moved relative to one another to effect engagement of the mold with the end plate and, thereby, force the end plate inwardly of the open-end of the injection cylinder. Such action will reduce the axial extent of the injection chamber, and will, thereby, in turn, cause the expulsion of elastomeric stock through the apertures in the end plate and into corresponding ones of the mold cavities.
It has been determined that in order to effectively produce elastomeric molded articles that are "flashless," i.e., from which there are absent burrs or other undesirable vulcanized apendages, it is important, among other things, to evacuate all undesirable fluids, such as air, entrapped in the mold cavities, as well as in the injection chamber communicating with the mold cavities via the apertures in the end plate, preparatory to filling such cavities with the elastomeric stock. The present invention is particularly concerned with the provision of means for eliminating the cause of molding imperfect articles as a result of the presence of air or other fluid entrapped in the mold cavities and the injection chamber.
A number of conventional systems have been suggested for evacuating fluid or air entrapped in the mold cavities, such as, for example, the systems disclosed in British Pat. No. 1,403,165, published on Aug. 13, 1975, and German Application No. 2,022,686, published on Dec. 2, 1971. One disadvantage associated with each of the British and German published disclosures is that in each instance the mold cavities are directly associated with a venting hose or tube and a vacuum pump for evacuating fluid entrapped in the mold cavities. As a result, because of the direct connection of the pump with the mold cavities, the pump creates a suction on the elastomeric stock in the mold cavities tending to pull at least a portion of the elastomeric stock from the mold cavities and into the vacuumed passageways, thus resulting in the formation of a suction wate plug. Not only is the formation of such waste costly, but it is also difficult to remove from the tubes or other passageways that they clog.